WASHINGTON, D.C.  What do TV's Big Bird, the Grateful Dead's Mickey Hart, basketball legend Larry Bird, and Gen. Colin Powell have in common? They were among those honored yesterday as ''living legends'' at the 200th birthday party for the Library of Congress, America's oldest federal cultural institution and the largest library in the world.

Pete Seeger sang "Blowin' in the Wind," percussionist Tito Puente played "Manteca" with saxophonist David Sanchez, and Dianne Reeves sang "Rocks in My Head" backed by the Saturday Night Live band on the grounds of the Capitol to help celebrate the party.

"I can't believe how well the grass is kept out here," Adam Ellgren, 27, a student at Georgetown Law School said. "The music sounds great, no one is being rude, and the weather for a change is really nice. I'm glad I skipped my contracts class at law school."

Yale University Historian Jaroslav Pelikan, spokesperson for the "living legends," said, "I seem to be the only one among those present whose last name puts him into the same class with Big Bird."

Big Bird, who is actually Carroll Spinney, joined the others on a stage set up on the grounds across the street from the library. He wore the familiar 8-foot fuzzy yellow costume, completed with a red-and-blue striped necktie. After blowing out the candles on a birthday cake in the shape of the Jefferson Memorial, Big Bird said ''D and C are two of my favorite letters. Gee, I've never been to a birthday party for a library before."

Following Big Bird, magician David Copperfield made himself disappear into a shrinking little box before, alas, returning to the shocked, screaming crowd. "How did he do that? I'm confused," William "Deke" Canada, from Washington, said.

Hart, who served as emcee, received a lifetime-achievement award not only for his contributions to rock 'n' roll with the Grateful Dead but also for his ethnomusicological contributions to the library.

"Any act that gives voice to the magic of music is a worthwhile pursuit," Hart said.